Automatic initiation of reference spectra library generation for optical monitoring

ABSTRACT

A method of generating reference spectra includes polishing a first substrate in a polishing apparatus, measuring a sequence of spectra from the first substrate during polishing with an in-situ optical monitoring system, for each spectrum in the sequence of spectra, determining a best matching reference spectrum from a first plurality of first reference spectra to generate a sequence of reference spectra, calculating a value of a metric of fit of the sequence of spectra to the sequence of reference spectra, comparing the value of the metric of fit to a threshold value and determining whether to generate a second library based on the comparison, and if the second library is determined to be generated, storing the sequence of spectra as a second plurality of reference spectra.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Application Ser. No. 61/352,325, filed on Jun. 7, 2010, the entiredisclosure of which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to the creation of referencespectra for optical monitoring, e.g., during chemical mechanicalpolishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier head. The exposed surface of thesubstrate is typically placed against a rotating polishing pad with adurable roughened surface. The carrier head provides a controllable loadon the substrate to push it against the polishing pad. A polishingliquid, such as a slurry with abrasive particles, is typically suppliedto the surface of the polishing pad.

One problem in CMP is using an appropriate polishing rate to achieve adesirable profile, e.g., a substrate layer that has been planarized to adesired flatness or thickness, or a desired amount of material has beenremoved. Variations in the initial thickness of a substrate layer, theslurry composition, the polishing pad condition, the relative speedbetween the polishing pad and a substrate, and the load on a substratecan cause variations in the material removal rate across a substrate,and from substrate to substrate. These variations cause variations inthe time needed to reach the polishing endpoint and the amount removed.Therefore, it may not be possible to determine the polishing endpointmerely as a function of the polishing time, or to achieve a desiredprofile merely by applying a constant pressure.

In some systems, a substrate is optically monitored in-situ duringpolishing, e.g., through a window in the polishing pad. However,existing optical monitoring techniques may not satisfy increasingdemands of semiconductor device manufacturers.

SUMMARY

In one aspect, a computer-implemented method of generating referencespectra includes polishing a first substrate in a polishing apparatus,measuring a sequence of spectra from the first substrate duringpolishing with an in-situ optical monitoring system, for each spectrumin the sequence of spectra, determining a best matching referencespectrum from a first plurality of first reference spectra to generate asequence of reference spectra, calculating a value of a metric of fit ofthe sequence of spectra to the sequence of reference spectra, comparingthe value of the metric of fit to a threshold value and determiningwhether to generate a second library based on the comparison, and if thesecond library is determined to be generated, storing the sequence ofspectra as a second plurality of reference spectra.

Implementations may include one or more of the following features. Asecond substrate may be polished in the polishing apparatus, a secondsequence of spectra may be measured from the second substrate duringpolishing with the in-situ monitoring system, and for each spectrum inthe second sequence of spectra, a best matching reference spectrum maybe determined from a set of reference spectra including the secondplurality of reference spectra to generate a second sequence ofreference spectra. The set of reference spectra may include the firstplurality of reference spectra. The set of reference spectra may notinclude the first plurality of reference spectra. Each referencespectrum in the second plurality of reference spectra may be associatedwith an index value proportional to a number of platen rotation at whichthe spectrum was measured. For each spectrum in the second sequence ofreference spectra, an associated index value may be determined togenerate a sequence of index values, and a linear function may be fit tothe sequence of index values. A target index value may be determined.The steps of measuring a second sequence of spectra, determining a bestmatching reference spectrum, determining an associated index value andfitting a linear function may be performed for a plurality of zones ofthe second substrate. A projected time at which at least one zone of thesecond substrate will reach the target index value may be determinedbased on the linear function. A polishing parameter for the at least onezone of the second substrate may be adjusted to adjust the polishingrate of the at least one zone such that the at least one zone has closerto the target index value at the projected time than without suchadjustment. An endpoint of polishing of the second substrate may bedetermined based on a time that the linear function reaches a targetindex value. Determining the target index value may include calculatingthe target index value using at least a first value. The first value maybe a number of platen rotations at which an endpoint was detected for athird substrate that was polished prior to the first substrate.Calculating the target index value may include calculating an average ofa plurality of values including the first value and a second value. Thesecond value may be a number of platen rotations at which an endpointwas detected for a fourth substrate that was polished prior to the thirdsubstrate. The fourth substrate, third substrate and first substrate maybe polished consecutively. The first substrate may be monitored with asecond in-situ monitoring system other than the optical monitoringsystem. A polishing endpoint of the first substrate may be detected withthe second in-situ monitoring system. Determining the target index valuemay include determining the number of platen rotations at the time thesecond in-situ monitoring system detects the polishing endpoint of thefirst substrate. The second in-situ monitoring system may include one ormore of a motor torque monitoring system, an eddy current monitoringsystem, a friction monitoring system, or a monochromatic opticalmonitoring system. post-polish thickness measurement of the firstsubstrate may be performed. An initial index value may be determined,and determining the target index value may include adjusting the initialindex value based on the post-polish thickness measurement. For eachspectrum in the first sequence of reference spectra, an associated indexvalue may be determined to generate a sequence of index values. A linearfunction may be fit to the sequence of index values. The metric of fitmay include a goodness of fit of the linear function to the sequence ofindex values. The goodness of fit may include a sum of squareddifferences between the sequence of index values and the linearfunction. Calculating the value of the metric of fit may include foreach spectrum in the sequence of spectra, determining a differencebetween the spectrum and the best matching reference spectrum to providea sequence of differences, and accumulating the differences. Determininga difference between the spectrum and the best matching referencespectrum may includes calculating a sum of absolute value of differencesin intensities over a range of wavelengths or calculating a sum ofsquare differences in intensities over a range of wavelengths.

In other aspects, polishing systems and computer-program productstangibly embodied on a computer readable medium are provided to carryout these methods.

Certain implementations may have one or more of the followingadvantages. Creation of reference spectra and a target index value canbe triggered automatically, thus permitting generation of a newreference library which can be used for polishing of subsequentsubstrates (e.g., subsequent substrates of the same lot). Adding orswitching to a new library when a prior library does not provide a goodfit can improving reliability and accuracy of the endpoint determinationand/or reduce within-wafer non-uniformity. Time required by thesemiconductor foundry to begin polishing of a new device substrate(e.g., a substrate generated based on a new mask pattern) can bereduced, and the need for different preset algorithms for eachdevice/mask pattern can be eliminated.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,aspects, and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of an example of apolishing apparatus having two polishing heads.

FIG. 2 illustrates a schematic top view of a substrate having multiplezones.

FIG. 3A illustrates a top view of a polishing pad and shows locationswhere in-situ measurements are taken on a first substrate.

FIG. 3B illustrates a top view of a polishing pad and shows locationswhere in-situ measurements are taken on a second substrate.

FIG. 4 illustrates a measured spectrum from the in-situ opticalmonitoring system.

FIG. 5 illustrates a library of reference spectra.

FIG. 6 illustrates an index trace.

FIG. 7 illustrates a plurality of index traces for different zones ofdifferent substrates.

FIG. 8 illustrates a calculation of a plurality of desired slopes for aplurality of adjustable zones based on a time that an index trace of areference zone reaches a target index.

FIG. 9 illustrates a calculation of a plurality of desired slopes for aplurality of adjustable zones based on a time that an index trace of areference zone reaches a target index.

FIG. 10 illustrates a plurality of index traces for different zones ofdifferent substrates, with different zones having different targetindexes.

FIG. 11 is a flow diagram of an example process for adjusting thepolishing rate of a a plurality of zones in a plurality of substratessuch that the plurality of zones have approximately the same thicknessat the target time.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

For optical monitoring systems used to monitor the spectra of reflectedlight from a substrate undergoing polishing, creation of referencespectra and a targets can be time-consuming. However, creation of a newlibrary of reference spectra can be triggered automatically. Forexample, if the reference spectra of a library does not provide a goodfit during polishing of a substrate, e.g., the fit is beyond somethreshold value, then the measured spectra from that substrate can bestored as the reference spectra of a new library. Thereafter, opticalmonitoring of subsequent substrates can proceed using the new library.

FIG. 1 illustrates an example of a polishing apparatus 100. Thepolishing apparatus 100 includes a rotatable disk-shaped platen 120 onwhich a polishing pad 110 is situated. The platen is operable to rotateabout an axis 125. For example, a motor 121 can turn a drive shaft 124to rotate the platen 120. The polishing pad 110 can be detachablysecured to the platen 120, for example, by a layer of adhesive. Thepolishing pad 110 can be a two-layer polishing pad with an outerpolishing layer 112 and a softer backing layer 114.

The polishing apparatus 100 can include a combined slurry/rinse arm 130.During polishing, the arm 130 is operable to dispense a polishing liquid132, such as a slurry, onto the polishing pad 110. While only oneslurry/rinse arm 130 is shown, additional nozzles, such as one or morededicated slurry arms per carrier head, can be used. The polishingapparatus can also include a polishing pad conditioner to abrade thepolishing pad 110 to maintain the polishing pad 110 in a consistentabrasive state.

In this implementation, the polishing apparatus 100 includes two (or twoor more) carrier heads 140. Each carrier head 140 is operable to hold asubstrate 10 (e.g., a first substrate 10 a at one carrier head and asecond substrate 10 b at the other carrier head) against the polishingpad 110, i.e., the same polishing pad. Each carrier head 140 can haveindependent control of the polishing parameters, for example pressure,associated with each respective substrate. In some implementations, thepolishing apparatus 100 includes multiple carrier heads, but the carrierheads (and the substrates held) are located over different polishingpads rather than the same polishing pad. For such implementations, thediscussion below of obtaining simultaneous endpoint of multiplesubstrates on the same platen does not apply, but the discussion ofobtaining simultaneous endpoint of multiple zones (albeit on a singlesubstrate) would still be applicable.

In particular, each carrier head 140 can include a retaining ring 142 toretain the substrate 10 below a flexible membrane 144. Each carrier head140 also includes a plurality of independently controllablepressurizable chambers defined by the membrane, e.g., 3 chambers 146a-146 c, which can apply independently controllable pressurizes toassociated zones 148 a-148 c on the flexible membrane 144 and thus onthe substrate 10 (see FIG. 2). Referring to FIG. 2, the center zone 148a can be substantially circular, and the remaining zones 148 b-148 e canbe concentric annular zones around the center zone 148 a. Although onlythree chambers are illustrated in FIGS. 1 and 2 for ease ofillustration, there could be two chambers, or four or more chambers,e.g., five chambers.

Returning to FIG. 1, each carrier head 140 is suspended from a supportstructure 150, e.g., a carousel, and is connected by a drive shaft 152to a carrier head rotation motor 154 so that the carrier head can rotateabout an axis 155. Optionally each carrier head 140 can oscillatelaterally, e.g., on sliders on the carousel 150; or by rotationaloscillation of the carousel itself. In operation, the platen is rotatedabout its central axis 125, and each carrier head is rotated about itscentral axis 155 and translated laterally across the top surface of thepolishing pad.

While only two carrier heads 140 are shown, more carrier heads can beprovided to hold additional substrates so that the surface area ofpolishing pad 110 may be used efficiently. Thus, the number of carrierhead assemblies adapted to hold substrates for a simultaneous polishingprocess can be based, at least in part, on the surface area of thepolishing pad 110.

The polishing apparatus also includes an in-situ monitoring system 160,which can be used to determine whether to adjust a polishing rate or anadjustment for the polishing rate as discussed below. The in-situmonitoring system 160 can include an optical monitoring system, e.g., aspectrographic monitoring system, or an eddy current monitoring system.

In one embodiment, the monitoring system 160 is an optical monitoringsystem. An optical access through the polishing pad is provided byincluding an aperture (i.e., a hole that runs through the pad) or asolid window 118. The solid window 118 can be secured to the polishingpad 110, e.g., as a plug that fills an aperture in the polishing pad,e.g., is molded to or adhesively secured to the polishing pad, althoughin some implementations the solid window can be supported on the platen120 and project into an aperture in the polishing pad.

The optical monitoring system 160 can include a light source 162, alight detector 164, and circuitry 166 for sending and receiving signalsbetween a remote controller 190, e.g., a computer, and the light source162 and light detector 164. One or more optical fibers can be used totransmit the light from the light source 162 to the optical access inthe polishing pad, and to transmit light reflected from the substrate 10to the detector 164. For example, a bifurcated optical fiber 170 can beused to transmit the light from the light source 162 to the substrate 10and back to the detector 164. The bifurcated optical fiber an include atrunk 172 positioned in proximity to the optical access, and twobranches 174 and 176 connected to the light source 162 and detector 164,respectively.

In some implementations, the top surface of the platen can include arecess 128 into which is fit an optical head 168 that holds one end ofthe trunk 172 of the bifurcated fiber. The optical head 168 can includea mechanism to adjust the vertical distance between the top of the trunk172 and the solid window 118.

The output of the circuitry 166 can be a digital electronic signal thatpasses through a rotary coupler 129, e.g., a slip ring, in the driveshaft 124 to the controller 190 for the optical monitoring system.Similarly, the light source can be turned on or off in response tocontrol commands in digital electronic signals that pass from thecontroller 190 through the rotary coupler 129 to the optical monitoringsystem 160. Alternatively, the circuitry 166 could communicate with thecontroller 190 by a wireless signal.

The light source 162 can be operable to emit white light. In oneimplementation, the white light emitted includes light havingwavelengths of 200-800 nanometers. A suitable light source is a xenonlamp or a xenon mercury lamp.

The light detector 164 can be a spectrometer. A spectrometer is anoptical instrument for measuring intensity of light over a portion ofthe electromagnetic spectrum. A suitable spectrometer is a gratingspectrometer. Typical output for a spectrometer is the intensity of thelight as a function of wavelength (or frequency).

As noted above, the light source 162 and light detector 164 can beconnected to a computing device, e.g., the controller 190, operable tocontrol their operation and receive their signals. The computing devicecan include a microprocessor situated near the polishing apparatus,e.g., a programmable computer. With respect to control, the computingdevice can, for example, synchronize activation of the light source withthe rotation of the platen 120.

In some implementations, the light source 162 and detector 164 of thein-situ monitoring system 160 are installed in and rotate with theplaten 120. In this case, the motion of the platen will cause the sensorto scan across each substrate. In particular, as the platen 120 rotates,the controller 190 can cause the light source 162 to emit a series offlashes starting just before and ending just after each substrate 10passes over the optical access. Alternatively, the computing device cancause the light source 162 to emit light continuously starting justbefore and ending just after each substrate 10 passes over the opticalaccess. In either case, the signal from the detector can be integratedover a sampling period to generate spectra measurements at a samplingfrequency.

In operation, the controller 190 can receive, for example, a signal thatcarries information describing a spectrum of the light received by thelight detector for a particular flash of the light source or time frameof the detector. Thus, this spectrum is a spectrum measured in-situduring polishing.

As shown by in FIG. 3A, if the detector is installed in the platen, dueto the rotation of the platen (shown by arrow 204), as the window 108travels below one carrier head (e.g., the carrier head holding the firstsubstrate 10 a), the optical monitoring system making spectrameasurements at a sampling frequency will cause the spectra measurementsto be taken at locations 201 in an arc that traverses the firstsubstrate 10 a. For example, each of points 201 a-201 k represents alocation of a spectrum measurement by the monitoring system of the firstsubstrate 10 a (the number of points is illustrative; more or fewermeasurements can be taken than illustrated, depending on the samplingfrequency). As shown, over one rotation of the platen, spectra areobtained from different radii on the substrate 10 a. That is, somespectra are obtained from locations closer to the center of thesubstrate 10 a and some are closer to the edge. Similarly, as shown byin FIG. 3B, due to the rotation of the platen, as the window travelsbelow the other carrier head (e.g., the carrier head holding the secondsubstrate 10 b) the optical monitoring system making spectrameasurements at the sampling frequency will cause the spectrameasurements to be taken at locations 202 along an arc that traversesthe second substrate 10 b.

Thus, for any given rotation of the platen, based on timing and motorencoder information, the controller can determine which substrate, e.g.,substrate 10 a or 10 b, is the source of the measured spectrum. Inaddition, for any given scan of the optical monitoring system across asubstrate, e.g., substrate 10 a or 10 b, based on timing, motor encoderinformation, and optical detection of the edge of the substrate and/orretaining ring, the controller 190 can calculate the radial position(relative to the center of the particular substrate 10 a or 10 b beingscanned) for each measured spectrum from the scan. The polishing systemcan also include a rotary position sensor, e.g., a flange attached to anedge of the platen that will pass through a stationary opticalinterrupter, to provide additional data for determination of whichsubstrate and the position on the substrate of the measured spectrum.The controller can thus associate the various measured spectra with thecontrollable zones 148 b-148 e (see FIG. 2) on the substrates 10 a and10 b. In some implementations, the time of measurement of the spectrumcan be used as a substitute for the exact calculation of the radialposition.

Over multiple rotations of the platen, for each zone of each substrate,a sequence of spectra can be obtained over time. Without being limitedto any particular theory, the spectrum of light reflected from thesubstrate 10 evolves as polishing progresses (e.g., over multiplerotations of the platen, not during a single sweep across the substrate)due to changes in the thickness of the outermost layer, thus yielding asequence of time-varying spectra. Moreover, particular spectra areexhibited by particular thicknesses of the layer stack.

In some implementations, the controller, e.g., the computing device, canbe programmed to compare a measured spectrum to multiple referencespectra to and determine which reference spectrum provides the bestmatch. In particular, the controller can be programmed to compare eachspectrum from a sequence of measured spectra from each zone of eachsubstrate to multiple reference spectra to generate a sequence of bestmatching reference spectra for each zone of each substrate.

As used herein, a reference spectrum is a predefined spectrum generatedprior to polishing of the substrate. A reference spectrum can have apre-defined association, i.e., defined prior to the polishing operation,with a value representing a time in the polishing process at which thespectrum is expected to appear, assuming that the actual polishing ratefollows an expected polishing rate. Alternatively or in addition, thereference spectrum can have a pre-defined association with a value of asubstrate property, such as a thickness of the outermost layer.

A reference spectrum can be generated empirically, e.g., by measuringthe spectra from a test substrate, e.g., a test substrate having a knowninitial layer thicknesses. For example, to generate a plurality ofreference spectra, a set-up substrate is polished using the samepolishing parameters that would be used during polishing of devicewafers while a sequence of spectra are collected. For each spectrum, avalue is recorded representing the time in the polishing process atwhich the spectrum was collected. For example, the value can be anelapsed time, or a number of platen rotations. The substrate can beoverpolished, i.e., polished past a desired thickness, so that thespectrum of the light that reflected from the substrate when the targetthickness is achieved can be obtained.

In order to associate each spectrum with a value of a substrateproperty, e.g., a thickness of the outermost layer, the initial spectraand property of a “set-up” substrate with the same pattern as theproduct substrate can be measured pre-polish at a metrology station. Thefinal spectrum and property can also be measured post-polish with thesame metrology station or a different metrology station. The propertiesfor spectra between the initial spectra and final spectra can bedetermined by interpolation, e.g., linear interpolation based on elapsedtime at which the spectra of the test substrate was measured.

In addition to being determined empirically, some or all of thereference spectra can be calculated from theory, e.g., using an opticalmodel of the substrate layers. For example, and optical model can beused to calculate a reference spectrum for a given outer layer thicknessD. A value representing the time in the polishing process at which thereference spectrum would be collected can be calculated, e.g., byassuming that the outer layer is removed at a uniform polishing rate.For example, the time Ts for a particular reference spectrum can becalculated simply by assuming a starting thickness D0 and uniformpolishing rate R (Ts=(D0−D)/R). As another example, linear interpolationbetween measurement times T1, T2 for the pre-polish and post-polishthicknesses D1, D2 (or other thicknesses measured at the metrologystation) based on the thickness D used for the optical model can beperformed (Ts=T2−T1*(D1−D)/(D1−D2)).

Referring to FIGS. 4 and 5, during polishing, a measured spectrum 300(see FIG. 4) can be compared to reference spectra 320 from one or morelibraries 310 (see FIG. 5). As used herein, a library of referencespectra is a collection of reference spectra which represent substratesthat share a property in common. However, the property shared in commonin a single library may vary across multiple libraries of referencespectra. For example, two different libraries can include referencespectra that represent substrates with two different underlyingthicknesses. For a given library of reference spectra, variations in theupper layer thickness, rather than other factors (such as differences inwafer pattern, underlying layer thickness, or layer composition), canprimarily responsible for the differences in the spectral intensities.

Reference spectra 320 for different libraries 310 can be generated bypolishing multiple “set-up” substrates with different substrateproperties (e.g., underlying layer thicknesses, or layer composition)and collecting spectra as discussed above; the spectra from one set-upsubstrate can provide a first library and the spectra from anothersubstrate with a different underlying layer thickness can provide asecond library. Alternatively or in addition, reference spectra fordifferent libraries can be calculated from theory, e.g., spectra for afirst library can be calculated using the optical model with theunderlying layer having a first thickness, and spectra for a secondlibrary can be calculated using the optical model with the underlyinglayer having a different one thickness.

In some implementations, each reference spectrum 320 is assigned anindex value 330. In general, each library 310 can include many referencespectra 320, e.g., one or more, e.g., exactly one, reference spectra foreach platen rotation over the expected polishing time of the substrate.This index 330 can be the value, e.g., a number, representing the timein the polishing process at which the reference spectrum 320 is expectedto be observed. The spectra can be indexed so that each spectrum in aparticular library has a unique index value. The indexing can beimplemented so that the index values are sequenced in an order in whichthe spectra were measured. An index value can be selected to changemonotonically, e.g., increase or decrease, as polishing progresses. Inparticular, the index values of the reference spectra can be selected sothat they form a linear function of time or number of platen rotations(assuming that the polishing rate follows that of the model or testsubstrate used to generate the reference spectra in the library). Forexample, the index value can be proportional, e.g., equal, to a numberof platen rotations at which the reference spectra was measured for thetest substrate or would appear in the optical model. Thus, each indexvalue can be a whole number. The index number can represent the expectedplaten rotation at which the associated spectrum would appear.

The reference spectra and their associated index values can be stored ina reference library. For example, each reference spectrum 320 and itsassociated index value 330 can be stored in a record 340 of database350. The database 350 of reference libraries of reference spectra can beimplemented in memory of the computing device of the polishingapparatus.

In some implementations, the reference spectra can be generatedautomatically for a given lot of substrates. The first substrate of alot, or the first substrate having a new device/mask pattern, ispolished while the optical monitoring system measures spectra, butwithout control of the polishing rate (discussed below with reference toFIGS. 8-10). This generates a sequence of spectra for the firstsubstrate, with at least one spectrum per zone per sweep of the windowbelow the substrate, e.g., per platen rotation.

A set of reference spectra, e.g., for each zone, is automaticallygenerated from the sequence of spectra for this first substrate. Inbrief, the spectra measured from the first substrate become thereference spectra. More particularly, the spectra measured from eachzone of the first substrate become the reference spectra for that zone.Each reference spectrum is associated with the platen rotation number atwhich it was measured from the first substrate. If there are multiplemeasured spectra for a particular zone of the first substrate at aparticular platen rotation, then the measured spectra can be combined,e.g., averaged to generate an average spectrum for that platen rotation.Alternatively, the reference library can simply keep each spectrum as aseparate reference spectrum, and compare the measured spectrum of thesubsequent substrate against each reference spectrum to find the bestmatch, as described below. Optionally, the database can store a defaultset of reference spectra, which are then replaced by the set ofreference spectra is generated from the sequence of spectra from thefirst substrate.

As noted above, the target index value can also be generatedautomatically. In some implementations, the first substrate is polishedfor a fixed polishing time, and the platen rotation number at the end ofthe fixed polishing time can be set as the target index value. In someimplementations, instead of a fixed polishing time, some form ofwafer-to-wafer feedforward or feedback control from the factory host orCMP tool (e.g., as described in U.S. application Ser. No. 12/625,480,incorporated by reference) can be used to adjust the polishing time forthe first wafer. The platen rotation number at the end of the adjustedpolishing time can be set as the target index value.

In some implementations, as shown in FIG. 1, the polishing system caninclude another endpoint detection system 180 (other than thespectrographic optical monitoring system 160), e.g., using frictionmeasurement (e.g., as described in U.S. Pat. No. 7,513,818, incorporatedby reference), eddy current (e.g., as described in U.S. Pat. No.6,924,641, incorporated by reference), motor torque (e.g., as describedin U.S. Pat. No. 5,846,882, incorporated by reference, or monochromaticlight, e.g., a laser (e.g., as described in U.S. Pat. No. 6,719,818,incorporated by reference). The other endpoint detection system 180 canbe in a separate recess 129 in the platen, or in the same recess 128 asthe optical monitoring system 160. In addition, although illustrated inFIG. 1 as on the opposite side of the axis of rotation of the platen125, this is not necessary, although the sensor of the endpointdetection system 180 can have the same radial distance from the axis 125as the optical monitoring system 160. This other endpoint detectionsystem 180 can be used to detect the polishing endpoint of the firstsubstrate, and the platen rotation number at the time that the otherendpoint detection system detects the endpoint can be set as the targetindex value. In some implementations, a post-polish thicknessmeasurement of the first substrate can be made, and an initial targetindex value as determined by one of the techniques above can beadjusted, e.g., by linear scaling, e.g., by multiplying by the ratio ofthe target thickness to the post-polish measured thickness.

In addition, the target index value can be further refined based on newsubstrates processed and the new desired endpoint time. In someimplementations, rather than using just the first substrate to set thetarget index value, the target index can be dynamically determined basedon a multiple previously polished substrates, e.g., by combining, e.g.,weighted averaging, of the endpoint times indicated by thewafer-to-wafer feedforward or feedback control or the other endpointdetection systems. A predefined number of the previously polishedsubstrates, e.g., four or less, that were polished immediately prior tothe present substrate, can be used in the calculation.

In any event, once a target index value has been determined, one or moresubsequent substrates can be polished using the techniques describedbelow to adjust the pressure applied to one or more zones so that thezones reach the target index at closer to the same time (or at anexpected endpoint time, are closer to their target index) than withoutsuch adjustment.

As noted above, for each zone of each substrate, based on the sequenceof measured spectra or that zone and substrate, the controller 190 canbe programmed to generate a sequence of best matching spectra. A bestmatching reference spectrum can be determined by comparing a measuredspectrum to the reference spectra from a particular library.

In some implementations, the best matching reference spectrum can bedetermined by calculating, for each reference spectra, a sum of squareddifferences between the measured spectrum and the reference spectrum.The reference spectrum with the lowest sum of squared differences hasthe best fit. Other techniques for finding a best matching referencespectrum are possible.

A method that can be applied to decrease computer processing is to limitthe portion of the library that is searched for matching spectra. Thelibrary typically includes a wider range of spectra than will beobtained while polishing a substrate. During substrate polishing, thelibrary searching is limited to a predetermined range of libraryspectra. In some embodiments, the current rotational index N of asubstrate being polished is determined. For example, in an initialplaten rotation, N can be determined by searching all of the referencespectra of the library. For the spectra obtained during a subsequentrotation, the library is searched within a range of freedom of N. Thatis, if during one rotation the index number is found to be N, during asubsequent rotation which is X rotations later, where the freedom is Y,the range that will be searched from (N+X)−Y to (N+X)+Y.

Referring to FIG. 6, which illustrates the results for only a singlezone of a single substrate, the index value of each of the best matchingspectra in the sequence can be determined to generate a time-varyingsequence of index values 212. This sequence of index values can betermed an index trace 210. In some implementations, an index trace isgenerated by comparing each measured spectrum to the reference spectrafrom exactly one library. In general, the index trace 210 can includeone, e.g., exactly one, index value per sweep of the optical monitoringsystem below the substrate.

For a given index trace 210, where there are multiple spectra measuredfor a particular substrate and zone in a single sweep of the opticalmonitoring system (termed “current spectra”), a best match can bedetermined between each of the current spectra and the reference spectraof one or more, e.g., exactly one, library. In some implementations,each selected current spectra is compared against each reference spectraof the selected library or libraries. Given current spectra e, f, and g,and reference spectra E, F, and G, for example, a matching coefficientcould be calculated for each of the following combinations of currentand reference spectra: e and E, e and F, e and G, f and E, f and F, fand G, g and E, g and F, and g and G. Whichever matching coefficientindicates the best match, e.g., is the smallest, determines thebest-matching reference spectrum, and thus the index value.Alternatively, in some implementations, the current spectra can becombined, e.g., averaged, and the resulting combined spectrum iscompared against the reference spectra to determine the best match, andthus the index value.

In some implementations, for at least some zones of some substrates, aplurality of index traces can be generated. For a given zone of a givensubstrate, an index trace can be generated for each reference library ofinterest. That is, for each reference library of interest to the givenzone of the given substrate, each measured spectrum in a sequence ofmeasured spectra is compared to reference spectra from a given library,a sequence of the best matching reference spectra is determined, and theindex values of the sequence of best matching reference spectra providethe index trace for the given library.

In summary, each index trace includes a sequence 210 of index values212, with each particular index value 212 of the sequence beinggenerated by selecting the index of the reference spectrum from a givenlibrary that is the closest fit to the measured spectrum. The time valuefor each index of the index trace 210 can be the same as the time atwhich the measured spectrum was measured.

Referring to FIG. 7, a plurality of index traces is illustrated. Asdiscussed above, an index trace can be generated for each zone of eachsubstrate. For example, a first sequence 210 of index values 212 (shownby hollow circles) can be generated for a first zone of a firstsubstrate, a second sequence 220 of index values 222 (shown by solidsquares) can be generated for a second zone of the first substrate, athird sequence 230 of index values 232 (shown by solid circles) can begenerated for a first zone of a second substrate, and a fourth sequence240 of index values 242 (shown by empty circles) can be generated for asecond zone of the second substrate.

As shown in FIG. 7, for each substrate index trace, a polynomialfunction of known order, e.g., a first-order function (e.g., a line) isfit to the sequence of index values for the associated zone and wafer,e.g., using robust line fitting. For example, a first line 214 can befit to index values 212 for the first zone of the first substrate, asecond line 224 can be fit to the index values 222 of the second zone ofthe first substrate, a third line 234 can be fit to the index values 232of the first zone of the second substrate, and a fourth line 244 can befit to the index values 242 of the second zone of the second substrate.Fitting of a line to the index values can include calculation of theslope S of the line and an x-axis intersection time T at which the linecrosses a starting index value, e.g., 0. The function can be expressedin the form I(t)=S·(t−T), where t is time. The x-axis intersection timeT can have a negative value, indicating that the starting thickness ofthe substrate layer is less than expected. Thus, the first line 214 canhave a first slope S1 and a first x-axis intersection time T1, thesecond line 224 can have a second slope S2 and a second x-axisintersection time T2, the third line 234 can have a third slope S3 and athird x-axis intersection time T3, and the fourth line 244 can have afourth slope S4 and a fourth x-axis intersection time T4.

Where multiple substrates are being polished simultaneously, e.g., onthe same polishing pad, polishing rate variations between the substratescan lead to the substrates reaching their target thickness at differenttimes. On the one hand, if polishing is halted simultaneously for thesubstrates, then some will not be at the desired thickness. On the otherhand, if polishing for the substrates is stopped at different times,then some substrates may have defects and the polishing apparatus isoperating at lower throughput.

By determining a polishing rate for each zone for each substrate fromin-situ measurements, a projected endpoint time for a target thicknessor a projected thickness for target endpoint time can be determined foreach zone for each substrate, and the polishing rate for at least onezone of at least one substrate can be adjusted so that the substratesachieve closer endpoint conditions. By “closer endpoint conditions,” itis meant that the zones of the substrates would reach their targetthickness closer to the same time than without such adjustment, or ifthe substrates halt polishing at the same time, that the zones of thesubstrates would have closer to the same thickness than without suchadjustment.

At some during the polishing process, e.g., at a time T0, a polishingparameter for at least one zone of at least one substrate, e.g., atleast one zone of every substrate, is adjusted to adjust the polishingrate of the zone of the substrate such that at a polishing endpointtime, the plurality of zones of the plurality of substrates are closerto their target thickness than without such adjustment. In someembodiments, each zone of the plurality of substrates can haveapproximately the same thickness at the endpoint time.

Referring to FIG. 8, in some implementations, one zone of one substrateis selected as a reference zone, and a projected endpoint time TE atwhich the reference zone will reach a target index IT is determined. Forexample, as shown in FIG. 8, the first zone of the first substrate isselected as the reference zone, although a different zone and/or adifferent substrate could be selected. The target thickness IT is set bythe user prior to the polishing operation and stored.

In order to determine the projected time at which the reference zonewill reach the target index, the intersection of the line of thereference zone, e.g., line 214, with the target index, IT, can becalculated. Assuming that the polishing rate does not deviate from theexpected polishing rate through the remainder polishing process, thenthe sequence of index values should retain a substantially linearprogression. Thus, the expected endpoint time TE can be calculated as asimple linear interpolation of the line to the target index IT, e.g.,IT=S·(TE−T). Thus, in the example of FIG. 8 in which the first zone ofthe second substrate is selected as the reference zone, with associatedthird line 234, IT=S1·(TE−T1), i.e., TE=IT/S1−T1.

One or more zones, e.g., all zones, other than the reference zone(including zones on other substrates) can be defined as adjustablezones. Where the lines for the adjustable zones meet the expectedendpoint time TE define projected endpoint for the adjustable zones. Thelinear function of each adjustable zone, e.g., lines 224, 234 and 244 inFIG. 8, can thus be used to extrapolate the index, e.g., EI2, EI3 andEI4, that will be achieved at the expected endpoint time ET for theassociated zone. For example, the second line 224 can be used toextrapolate the expected index, EI2, at the expected endpoint time ETfor the second zone of the first substrate, the third line 234 can beused to extrapolate the expected index, EI3, at the expected endpointtime ET for the first zone of the second substrate, and the fourth linecan be used to extrapolate the expected index, EI4, at the expectedendpoint time ET for the second zone of the second substrate.

As shown in FIG. 8, if no adjustments are made to the polishing rate ofany of the zones of any the substrates after time T0, then if endpointis forced at the same time for all substrates, then each substrate canhave a different thickness, or each substrate could have a differentendpoint time (which is not desirable because it can lead to defects andloss of throughput). Here, for example, the second zone of the firstsubstrate (shown by line 224) would endpoint at an expected index EI2greater (and thus a thickness less) than the expected index of the firstzone of the first substrate. Likewise, the first zone of the secondsubstrate would endpoint at an expected index ET3 less (and thus athickness greater) than the first zone of the first substrate.

If, as shown in FIG. 8, the target index will be reached at differenttimes for different substrates (or equivalently, the adjustable zoneswill have different expected indexes at the projected endpoint time ofreference zone), the polishing rate can be adjusted upwardly ordownwardly, such that the substrates would reach the target index (andthus target thickness) closer to the same time than without suchadjustment, e.g., at approximately the same time, or would have closerto the same index value (and thus same thickness), at the target timethan without such adjustment, e.g., approximately the same index value(and thus approximately the same thickness).

Thus, in the example of FIG. 8, commencing at a time T0, at least onepolishing parameter for the second zone of the first substrate ismodified so that the polishing rate of the zone is decreased (and as aresult the slope of the index trace 220 is decreased). Also, in thisexample, at least one polishing parameter for the first zone of thesecond substrate is modified so that the polishing rate of the zone isdecreased (and as a result the slope of the index trace 230 isdecreased). Similarly, in this example, at least one polishing parameterfor the second zone of the second substrate is modified so that thepolishing rate of the zone is decreased (and as a result the slope ofthe index trace 240 is decreased). As a result both zones of bothsubstrates would reach the target index (and thus the target thickness)at approximately the same time (or if polishing of both substrates haltsat the same time, both zones of both substrates will end withapproximately the same thickness).

In some implementations, if the projected index at the expected endpointtime ET indicate that a zone of the substrate is within a predefinedrange of the target thickness, then no adjustment may be required forthat zone. The range may be 2%, e.g., within 1%, of the target index.

The polishing rates for the adjustable zones can be adjusted so that allof the zones are closer to the target index at the expected endpointtime than without such adjustment. For example, a reference zone of thereference substrate might be chosen and the processing parameters forall of the other zone adjusted such that all of the zones will endpointat approximately the projected time of the reference substrate. Thereference zone can be, for example, a predetermined zone, e.g., thecenter zone 148 a or the zone 148 b immediately surrounding the centerzone, the zone having the earliest or latest projected endpoint time ofany of the zones of any of the substrates, or the zone of a substratehaving the desired projected endpoint. The earliest time is equivalentto the thinnest substrate if polishing is halted at the same time.Likewise, the latest time is equivalent to the thickest substrate ifpolishing is halted at the same time. The reference substrate can be,for example, a predetermined substrate, a substrate having the zone withthe earliest or latest projected endpoint time of the substrates. Theearliest time is equivalent to the thinnest zone if polishing is haltedat the same time. Likewise, the latest time is equivalent to thethickest zone if polishing is halted at the same time.

For each of the adjustable zones, a desired slope for the index tracecan be calculated such that the adjustable zone reaches the target indexat the same time as the reference zone. For example, the desired slopeSD can be calculated from (IT−I)=SD*(TE−T0), where I is the index value(calculated from the linear function fit to the sequence of indexvalues) at time T0 polishing parameter is to be changed, IT is thetarget index, and TE is the calculated expected endpoint time. In theexample of FIG. 8, for the second zone of the first substrate, thedesired slope SD2 can be calculated from (IT−I2)=SD2*(TE−T0), for thefirst zone of the second substrate, the desired slope SD3 can becalculated from (IT−I3)=SD3*(TE−T0), and for the second zone of thesecond substrate, the desired slope SD4 can be calculated from(IT−I4)=SD4*(TE−T0).

Referring to FIG. 9, in some implementations, there is no referencezone. For example, the expected endpoint time TE′ can be a predeterminedtime, e.g., set by the user prior to the polishing process, or can becalculated from an average or other combination of the expected endpointtimes of two or more zones (as calculated by projecting the lines forvarious zones to the target index) from one or more substrates. In thisimplementation, the desired slopes are calculated substantially asdiscussed above (using the expected endpoint time TE′ rather than TE),although the desired slope for the first zone of the first substratemust also be calculated, e.g., the desired slope SD1 can be calculatedfrom (IT−I1)=SD1*(TE′−T0).

Referring to FIG. 10, in some implementations, (which can also becombined with the implementation shown in FIG. 9), there are differenttarget indexes for different zones. This permits the creation of adeliberate but controllable non-uniform thickness profile on thesubstrate. The target indexes can be entered by user, e.g., using aninput device on the controller. For example, the first zone of the firstsubstrate can have a first target indexes IT1, the second zone of thefirst substrate can have a second target indexes IT2, the first zone ofthe second substrate can have a third target indexes IT3, and the secondzone of the second substrate can have a fourth target indexes IT4.

The desired slope SD for each adjustable zone can be calculated from(IT−I)=SD*(TE−T0), where I is the index value of the zone (calculatedfrom the linear function fit to the sequence of index values for thezone) at time T0 at which the polishing parameter is to be changed, ITis the target index of the particular zone, and TE is the calculatedexpected endpoint time (either from a reference zone as discussed abovein relation to FIG. 8, or from a preset endpoint time or from acombination of expected endpoint times as discussed above in relation toFIG. 9). In the example of FIG. 10, for the second zone of the firstsubstrate, the desired slope SD2 can be calculated from(IT2−I2)=SD2*(TE−T0), for the first zone of the second substrate, thedesired slope SD3 can be calculated from (IT3−I3)=SD3*(TE−T0), and forthe second zone of the second substrate, the desired slope SD4 can becalculated from (IT4−I4)=SD4*(TE−T0).

For any of the above methods described above for FIGS. 8-10, thepolishing rate is adjusted to bring the slope of index trace closer tothe desired slope. The polishing rates can be adjusted by, for example,increasing or decreasing the pressure in a corresponding chamber of acarrier head. The change in polishing rate can be assumed to be directlyproportional to the change in pressure, e.g., a simple Prestonian model.For example, for each zone of each substrate, where zone was polishedwith a pressure Pold prior to the time T0, a new pressure Pnew to applyafter time T0 can be calculated as Pnew=Pold*(SD/S), where S is theslope of the line prior to time T0 and SD is the desired slope.

For example, assuming that pressure Pold1 was applied to the first zoneof the first substrate, pressure Pold2 was applied to the second zone ofthe first substrate, pressure Pold3 was applied to the first zone of thesecond substrate, and pressure Pold4 was applied to the second zone ofthe second substrate, then new pressure Pnew1 for the first zone of thefirst substrate can be calculated as Pnew1=Pold1*(SD1/S1), the newpressure Pnew2 for the second zone of the first substrate clan becalculated as Pnew2=Pold2*(SD2/S2), the new pressure Pnew3 for the firstzone of the second substrate clan be calculated as Pnew3=Pold3*(SD3/S3),and the new pressure Pnew4 for the second zone of the second substrateclan be calculated as Pnew4=Pold4*(SD4/S4).

The process of determining projected times that the substrates willreach the target thickness, and adjusting the polishing rates, can beperformed just once during the polishing process, e.g., at a specifiedtime, e.g., 40 to 60% through the expected polishing time, or performedmultiple times during the polishing process, e.g., every thirty to sixtyseconds. At a subsequent time during the polishing process, the ratescan again be adjusted, if appropriate. During the polishing process,changes in the polishing rates can be made only a few times, such asfour, three, two or only one time. The adjustment can be made near thebeginning, at the middle or toward the end of the polishing process.

Polishing continues after the polishing rates have been adjusted, e.g.,after time T0, and the optical monitoring system continues to collectspectra and determine index values for each zone of each substrate. Oncethe index trace of a reference zone reaches the target index (e.g., ascalculated by fitting a new linear function to the sequence of indexvalues after time T0 and determining the time at which the new linearfunction reaches the target index), endpoint is called and the polishingoperation stops for both substrates. The reference zone used fordetermining endpoint can be the same reference zone used as describedabove to calculate the expected endpoint time, or a different zone (orif all of the zones were adjusted as described with reference to FIG. 8,then a reference zone can be selected for the purpose of endpointdetermination).

In some implementations, e.g., for copper polishing, after detection ofthe endpoint for a substrate, the substrate is immediately subjected toan overpolishing process, e.g., to remove copper residue. Theoverpolishing process can be at a uniform pressure for all zones of thesubstrate, e.g., 1 to 1.5 psi. The overpolishing process can have apreset duration, e.g., 10 to 15 seconds.

In some implementations, polishing of the substrates does not haltsimultaneously. In such implementations, for the purpose of the endpointdetermination, there can be a reference zone for each substrate. Oncethe index trace of a reference zone of a particular substrate reachesthe target index (e.g., as calculated by the time the linear functionfit the sequence of index values after time T0 reaches the targetindex), endpoint is called for the particular substrate and applicationof pressure to all zones of the particular is halted simultaneously.However, polishing of one or more other substrates can continue. Onlyafter endpoint has been called for the all of the remaining substrates(or after overpolishing has been completed for all substrates), based onthe reference zones of the remaining substrates, does rinsing of thepolishing pad commence. In addition, all of the carrier heads can liftthe substrates off the polishing pad simultaneously.

Where multiple index traces are generated for a particular zone andsubstrate, e.g., one index trace for each library of interest to theparticular zone and substrate, then one of the index traces can beselected for use in the endpoint or pressure control algorithm for theparticular zone and substrate. For example, the each index tracegenerated for the same zone and substrate, the controller 190 can fit alinear function to the index values of that index trace, and determine agoodness of fit of the that linear function to the sequence of indexvalues. The index trace generated having the line with the best goodnessof fit its own index values can be selected as the index trace for theparticular zone and substrate. For example, when determining how toadjust the polishing rates of the adjustable zones, e.g., at time T0,the linear function with the best goodness of fit can be used in thecalculation. As another example, endpoint can be called when thecalculated index (as calculated from the linear function fit to thesequence of index values) for the line with the best goodness of fitmatches or exceeds the target index. Also, rather than calculating anindex value from the linear function, the index values themselves couldbe compared to the target index to determine the endpoint.

Determining whether an index trace associated with a spectra library hasthe best goodness of fit to the linear function associated with thelibrary can include determining whether the index trace of theassociated spectra library has the least amount of difference from theassociated robust line, relatively, as compared to the differences fromthe associated robust line and index trace associated with anotherlibrary, e.g., the lowest standard deviation, the greatest correlation,or other measure of variance. In one implementation, the goodness of fitis determined by calculating a sum of squared differences between theindex data points and the linear function; the library with the lowestsum of squared differences has the best fit.

In some implementations, generation of a new library of referencespectra can be triggered automatically. The controller 190 can store athreshold value that will be used to determine whether to triggergeneration of a new library. For example, the threshold value can be setby the operator prior to polishing, or the threshold value can be apre-programmed value.

During polishing a sequence of measured spectra is obtained for asubstrate, e.g., for each zone of a substrate, e.g., using thetechniques described above. These measured spectra can be used forcontrol of the polishing rate (as discussed above with reference toFIGS. 8-10) and/or for control of the endpoint. For each measuredspectrum from the sequence, a reference spectrum from a preexistinglibrary of reference spectra is selected as the best matching spectrum,e.g., using the techniques described above, thus generating a sequenceof best matching spectra.

The controller can be programmed to calculate a value of a metric of fitof the sequence of best matching spectra to the sequence of measuredspectra. The value of the metric of fit is compared to the thresholdvalue. If the value of the metric of fit is beyond the threshold value(e.g., above or below, depending on whether a low or high value,respectively, of the metric of fit indicates a good fit), thengeneration of a new library of reference spectra is triggered.

In some implementations, the metric of fit is a goodness of fit of thelinear function to the index trace, e.g., the standard deviation, thecorrelation, or other measure of variance between the linear functionand the sequence of index values for the measured spectra. For example,the metric of fit can be a sum of squared differences between the indexdata points and the linear function. In some implementations, the metricof fit is based on an accumulated difference between the sequence ofmeasured spectra and the sequence of best matching reference spectra.For example, the metric of fit can determined by calculating adifference between each measured spectrum and the best-matchingreference spectrum for that measured spectrum, and summing thedifferences. Calculating the difference between a measured spectrum anda best matching spectrum can include calculating the difference as a sumof absolute value of differences in intensities over a range ofwavelengths. That is,difference=Σ^(B) _(λ=A)abs(I _(measured)(λ)−I _(reference)(λ))where A and are the lower limit and upper limit of the range ofwavelengths of a spectrum, respectively, and I_(measured)(λ) andI_(reference)(λ) are the intensity of a measured spectra and theintensity of the reference spectra for a given wavelength, respectively.Alternatively, calculating the difference between a measured spectrumand a best matching spectrum can include calculating the difference as asum of square differences in intensities over a range of wavelengths.That is:difference=Σ^(B) _(λ=A)(I _(measured)(λ)−I _(reference)(λ))²

In some implementations, the metric of fit is based on a differencebetween the endpoint time TE for the measured substrate and an averagedendpoint time for prior measured substrates.

If generation of a new library of reference spectra is triggered, thenthe set of measured spectra from the substrate becomes the new libraryof reference spectra. This can use similar techniques as the generationof a reference library from the first substrate of a lot as describedabove. In brief, an index value is generated for and associated witheach reference spectrum. The index value can be proportional, e.g.,equal, to the platen rotation number at which it was measured from thesubstrate. In addition, a new target index value can also be generatedautomatically. The new target index value can be calculated from theplaten rotation number at which endpoint was detected for one or priorsubstrates, e.g., for one or more substrates other than the substratethat triggered the generation of the new library. For example, the newtarget index value can be calculated from an average, e.g., a weightedaverage, of the platen rotation number at which endpoint was detectedfor the two or more consecutive substrates prior to the substrate thattriggered the generation of the new library. Alternatively, if thepolishing system include another endpoint detection system 180 (otherthan the spectrographic optical monitoring system 160), then the platenrotation number at the time that the other endpoint detection system 180detects the endpoint can be set as the target index value. In someimplementations, a post-polish thickness measurement of the firstsubstrate can be made, and an initial target index value as determinedby one of the techniques above can be adjusted, e.g., by linear scaling,e.g., by multiplying by the ratio of the target thickness to thepost-polish measured thickness.

The new library can be added to an existing set of one or morelibraries, or can replace an old library.

Referring to FIG. 11, a summary flow chart 600 is illustrated. Aplurality of zones of a plurality of substrates are polished in apolishing apparatus simultaneously with the same polishing pad (step602), as described above. During this polishing operation, each zone ofeach substrate has its polishing rate controllable independently of theother substrates by an independently variable polishing parameter, e.g.,the pressure applied by the chamber in carrier head above the particularzone. During the polishing operation, the substrates are monitored (step604) as described above, e.g., with a measured spectrum obtained fromeach zone of each substrate. The reference spectrum that is the bestmatch is determined (step 606). The index value for each referencespectrum that is the best fit is determined to generate sequence ofindex values (step 610). For each zone of each substrate, a linearfunction is fit to the sequence of index values (step 610). In oneimplementation, an expected endpoint time that the linear function for areference zone will reach a target index value is determined, e.g., bylinear interpolation of the linear function (step 612). In otherimplementations, the expected endpoint time is predetermined orcalculated as a combination of expected endpoint times of multiplezones. If needed, the polishing parameters for the other zones of theother substrates are adjusted to adjust the polishing rate of thatsubstrate such that the plurality of zones of the plurality ofsubstrates reach the target thickness at approximately the same time orsuch that the plurality of zones of the plurality of substrates haveapproximately the same thickness (or a target thickness) at the targettime (step 614). Polishing continues after the parameters are adjusted,and for each zone of each substrate, measuring a spectrum, determiningthe best matching reference spectrum from a library, determining theindex value for the best matching spectrum to generate a new sequence ofindex values for the time period after the polishing parameter has beenadjusted, and fitting a linear function to index values (step 616).Polishing can be halted once the index value for a reference zone (e.g.,a calculated index value generated from the linear function fit to thenew sequence of index values) reaches target index (step 630).

The techniques described above can also be applicable for monitoring ofmetal layers using an eddy current system. In this case, rather thanperforming matching of spectra, the layer thickness (or a valuerepresentative thereof) is measured directly by the eddy currentmonitoring system, and the layer thickness is used in place of the indexvalue for the calculations.

The method used to adjust endpoints can be different based upon the typeof polishing performed. For copper bulk polishing, a single eddy currentmonitoring system can be used. For copper-clearing CMP with multiplewafers on a single platen, a single eddy current monitoring system canfirst be used so that all of the substrates reach a first breakthroughat the same time. The eddy current monitoring system can then beswitched to a laser monitoring system to clear and over-polish thewafers. For barrier and dielectric CMP with multiple wafers on a singleplaten, an optical monitoring system can be used.

In some implementations, where the polishing system includes anotherendpoint detection system (other than the spectrographic system), thepressures of the zones can be adjusted using the techniques describedabove, but the actual endpoint can be detected by the other endpointdetection system. For example, for copper polishing, this permits thespectrographic monitoring system to reduce residue and overpolishing,but permits the other system, e.g., the motor torque sensor or frictionbased sensor, which can be more reliable in determination of thepolishing endpoint, to determine the polishing endpoint.

As used in the instant specification, the term substrate can include,for example, a product substrate (e.g., which includes multiple memoryor processor dies), a test substrate, a bare substrate, and a gatingsubstrate. The substrate can be at various stages of integrated circuitfabrication, e.g., the substrate can be a bare wafer, or it can includeone or more deposited and/or patterned layers. The term substrate caninclude circular disks and rectangular sheets.

The controller 190 can include a central processing unit (CPU) 192, amemory 194, and support circuits 196, e.g., input/output circuitry,power supplies, clock circuits, cache, and the like. In addition toreceiving signals from the optical monitoring system 160 (and any otherendpoint detection system 180), the controller 190 can be connected tothe polishing apparatus 100 to control the polishing parameters, e.g.,the various rotational rates of the platen(s) and carrier head(s) andpressure(s) applied by the carrier head. The memory is connected to theCPU 192. The memory, or computable readable medium, can be one or morereadily available memory such as random access memory (RAM), read onlymemory (ROM), floppy disk, hard disk, or other form of digital storage.In addition, although illustrated as a single computer, the controller190 could be a distributed system, e.g., including multipleindependently operating processors and memories.

Embodiments of the invention and all of the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructural means disclosed in this specification and structuralequivalents thereof, or in combinations of them. Embodiments of theinvention can be implemented as one or more computer program products,i.e., one or more computer programs tangibly embodied in amachine-readable storage media, for execution by, or to control theoperation of, data processing apparatus, e.g., a programmable processor,a computer, or multiple processors or computers. A computer program(also known as a program, software, software application, or code) canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment. A computer programdoes not necessarily correspond to a file. A program can be stored in aportion of a file that holds other programs or data, in a single filededicated to the program in question, or in multiple coordinated files(e.g., files that store one or more modules, sub-programs, or portionsof code). A computer program can be deployed to be executed on onecomputer or on multiple computers at one site or distributed acrossmultiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierheads, or both can move to provide relative motion between the polishingsurface and the substrate. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems, e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly. Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and substrate can beheld in a vertical orientation or some other orientation.

Particular embodiments of the invention have been described. Otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A computer-implemented method of generatingreference spectra, comprising: polishing a first substrate in apolishing apparatus; measuring a first sequence of spectra from thefirst substrate during polishing with an in-situ optical monitoringsystem; for each spectrum in the first sequence of spectra, determininga best matching reference spectrum from a first set of reference spectrato generate a first sequence of reference spectra, the first set ofreference spectra including a first plurality of reference spectra of afirst library generated prior to polishing of the first substrate;calculating a value of a metric of fit of the first sequence of spectrato the first sequence of reference spectra; comparing the value of themetric of fit to a threshold value and determining whether to generate asecond library based on the comparison; prior to polishing a secondsubstrate, if the second library is determined to be generated, storingthe first sequence of spectra as a second plurality of referencespectra; polishing a second substrate in the polishing apparatus;measuring a second sequence of spectra from the second substrate duringpolishing with the in-situ monitoring system; and for each spectrum inthe second sequence of spectra, determining a best matching referencespectrum from a second set of reference spectra to generate a secondsequence of reference spectra, the second set of reference spectraincluding the second plurality of reference spectra.
 2. The method ofclaim 1, wherein the second set of reference spectra includes the firstplurality of reference spectra.
 3. The method of claim 1, wherein thesecond set of reference spectra does not include the first plurality ofreference spectra.
 4. The method of claim 1, further comprisingassociating each reference spectrum in the second plurality of referencespectra with an index value proportional to a number of platen rotationat which the spectrum was measured.
 5. The method of claim 4, furthercomprising for each spectrum in the second sequence of referencespectra, determining an associated index value to generate a sequence ofindex values; and fitting a linear function to the sequence of indexvalues.
 6. The method of claim 5, further comprising determining atarget index value.
 7. The method of claim 6, wherein the steps ofmeasuring the second sequence of spectra, determining the best matchingreference spectrum, determining the associated index value and fittingthe linear function are performed for a plurality of zones of the secondsubstrate.
 8. The method of claim 7, further comprising: determining aprojected time at which at least one zone of the second substrate willreach the target index value based on the linear function; and adjustinga polishing parameter for the at least one zone of the second substrateto adjust the polishing rate of the at least one zone such that the atleast one zone has closer to the target index value at the projectedtime than without such adjustment.
 9. The method of claim 6, furthercomprising detecting an endpoint of polishing of the second substratebased on a time that the linear function reaches the target index value.10. The method of claim 6, wherein determining the target index valueincludes calculating the target index value using at least a firstvalue, the first value being a number of platen rotations at which anendpoint was detected for a third substrate that was polished prior tothe first substrate.
 11. The method of claim 10, wherein calculating thetarget index value includes calculating an average of a plurality ofvalues including the first value and a second value, the second valuebeing a number of platen rotations at which an endpoint was detected fora fourth substrate that was polished prior to the third substrate. 12.The method of claim 11, wherein the fourth substrate, third substrateand first substrate are polished consecutively.
 13. The method of claim6, further comprising monitoring the first substrate with a secondin-situ monitoring system other than the optical monitoring system, anddetecting a polishing endpoint of the first substrate with the secondin-situ monitoring system.
 14. The method of claim 13, whereindetermining the target index value includes determining a number ofplaten rotations at a time the second in-situ monitoring system detectsfor polishing endpoint of the first substrate.
 15. The method of claim14, wherein the second in-situ monitoring system comprises one or moreof a motor torque monitoring system, an eddy current monitoring system,a friction monitoring system, or a monochromatic optical monitoringsystem.
 16. The method of claim 6, further comprising performing apost-polish thickness measurement of the first substrate.
 17. The methodof claim 16, further comprising determining an initial index value, andwherein determining the target index value includes adjusting theinitial index value based on the post-polish thickness measurement. 18.The method of claim 1, further comprising: for each spectrum in thefirst sequence of reference spectra, determining an associated indexvalue to generate a sequence of index values; and fitting a linearfunction to the sequence of index values.
 19. The method of claim 1,wherein the metric of fit comprises a goodness of fit of the linearfunction to the sequence of index values.
 20. The method of claim 19,wherein the goodness of fit comprises a sum of squared differencesbetween the sequence of index values and the linear function.
 21. Themethod of claim 1, wherein calculating the value of the metric of fitcomprises for each spectrum in the first sequence of spectra,determining a difference between the spectrum and the best matchingreference spectrum from the first set of reference spectra to provide asequence of differences, and accumulating the differences.
 22. Themethod of claim 21, wherein determining the difference between thespectrum and the best matching reference spectrum includes calculating asum of absolute value of differences in intensities over a range ofwavelengths or calculating a sum of squared differences in intensitiesover a range of wavelengths.